Mapping the topography of the human cornea is facilitated by employing a Placido disk device which causes a predetermined pattern of illuminated rings to be reflected upon the cornea. While the conventional Placido disk fairly easily produces rings in the apical region of the cornea, to provide rings in the limbal region of the cornea it has until recently been necessary to employ a rather large diameter Placido disk. However, U.S. Pat. No. 4,772,115 disclosed a compact Placido disk device which provides a greater number of limbal rings without incurring the penalty of a large and bulky apparatus. Briefly, the device of that patent employed a conical structure made of light pervious plastic having an eye-port at one end of a substantially cylindrical central bore. The base of the cone was illuminated and a series of light-transmitting and opaque bands were arrayed along the central bore. The series of light transmitting and opaque bands were produced by first coating the central bore with a uniform opaque coating and then making a series of ring-shaped cuts through the coating to reveal the underlying light transmitting plastic material of the cone. The bore was illuminated by a lightbox attached to the base of the conical structure. While that device permitted the large and bulky Placido disk to be replaced by a more compact device that reflected an acceptable number of illuminated rings upon both the apical and limbal regions of the cornea, certain difficulties were encountered in its fabrication. A further improvement in the conical format keratometer was disclosed in U.S. Pat. No. 5,018,850 in which the light and dark bands for the illuminated bore were produced by first incising bands along the bore, filling the incised bands with opaque material and thereafter removing the excess opaque material from the lands. The latter method permitted a more accurate delineation of the edges of the opaque rings.
While the Placido disk devices fabricated according to the foregoing methods have been successfully employed in practice, the need to employ precise, multistep machine processing steps in both fabrication techniques has been found to be expensive and time consuming. A more economical method of producing a precisely defined illuminated pattern on the cornea would be desirable. In addition, it would be most advantageous to be able to cause other precisely defined illuminated patterns to be reflected upon the cornea such as, for example, radial lines as well as non-circular mire patterns.